U.S. Pat. No. 4,250,211 discloses a novel inverted blade type apparatus and paper coating method that has come to be known as the "short dwell time application" or "SDTA" method and apparatus. The SDTA coater has essentially revolutionized the paper coating art.
The present invention provides a new and improved coating apparatus and method which utilizes, in a specific non-conventional interrelationship, modifications of and improvements upon SDTA and other web coating technologies.
A conventional coater of the trailing blade type includes means for applying a liquid coating composition to a moving web of paper, usually while the web is supported and carried by a resilient backing roll, together with a doctor blade located on the trailing side of the applicator and bearing under pressure against the roll supported coated web to level the applied coating. In general, an excess of coating material is applied to the web, and the trailing blade then meters or removes the excess while uniformly spreading the retained coating onto the web surface.
A first generation of blade coating apparatus, known as the "pond" or "puddle" coater, is comprised essentially of a blade angled downwardly toward and contacting the backing roll on the downwardly moving, incoming side of the roll and forming therewith a reservoir for coating material. The web is moved on the backing roll continuously through the reservoir and the "pond" or "puddle" of coating material therein, whereupon the exposed surface of the web picks up coating material which is struck off and leveled to the desired final thickness or coat weight as a consequence of passage of the web through the nip defined between the blade and the backing roll. Examples of this type of coater are shown in Pulp & Paper, Apr. 29, 1963, pp. 56-58, Paper Trade Journal, Oct. 27, 1969, pp. 58-62 and Paper Trade Journal, Feb. 22, 1971, p. 56.
A variant on the pond type coater, publicized as the Kohler Coater, eliminates the backing roll, disposes the pond or puddle in the horizontal plane, moves the web across the surface of the pond, and utilizes a variable pressure air knife to press the paper web against the blade at the web outlet end of the pond. The Kohler Coater, which is not known to have gained commercial acceptance, is disclosed in Kohler, U.S. Pat. No. 3,113,884; Colgan U.S Pat. No. 3,083,685; and articles appearing in the June 1959 issue of The Paper Industry, p. 232; the Jun. 8, 1959 issue of Paper Trade Journal, pp. 31-32; the February 1960 issue of Tappi, pp. 183-187; Pulp and Paper, Second Edition, Vol. III, Interscience Publishers, pp. 1565-1566; and Pulp and Paper Manufacture, Second Edition, Vol. II, 1969, McGraw Hill Book Company, pp. 510-511.
A second generation of blade coating apparatus is comprised of a dip roll applicator, which usually bears against the roll supported web at or adjacent the bottom dead center position of the roll, and a blade spaced downstream from the dip roll and converging toward and contacting the roll supported web, usually on the upwardly moving, outgoing side of the roll. Since this results in the blade converging upwardly into engagement with the roll supported web, the blade is known as an inverted trailing blade. As the web moves with the backing roll, the dip roll is rotated through a reservoir of coating liquid and picks up and transfers to the web an excess of coating liquid. The web then travels to the inverted blade where the excess coating liquid is removed from the web and the retained coating is leveled to the desired final coat weight thickness. Examples of the dip roll applicator with inverted blade (known by the acronym "drib") are disclosed in Rush U.S. Pat. No. 2,746,877; Dickerman et al., U.S. Pat. No. 2,949,382, Brezinski U.S. Pat. No. 3,202,536, the Apr. 29, 1963 issue of Pulp & Paper, p. 57, and the Oct. 7, 1969 issue of Paper Trade Journal, pp. 60-61. In installations wherein a pool of coating liquid is accumulated at the nip between the two rolls, the coater may also be known as a "flooded nip" coater. Another version, involving the use of several applicator rolls in sequence, called the Champflex Coater, is disclosed at pages 56-57 of the Apr. 29, 1963 issue of Pulp & Paper. Also, dip roll applicators may be used in combination with other coaters for precoating or prewetting the web, as is shown for example in the illustration of the Kohler Coater in Pulp and Paper Manufacture, p. 511, and also in Damrau et al., U.S. Pat. No. 4,250,211 and Damrau U.S. Pat. No. 4,310,573.
A major shortcoming of dip roll coaters is the development of a film split pattern in the final coated web, i.e., the appearance in the coating of substantially continuous longitudinal stripes or lines, as web coating speeds are increased above 2,500 feet per minute and coatweights exceed about 51/2 bone dry pounds per side per 3,300 square foot ream.
A third generation of blade coater, called the flexible blade or "Flexiblade" Coater, is comprised of a closed, pressurized, coating application chamber which sealingly engages the roll supported web, usually near the bottom of the backing roll, and has a back, rear or outgoing wall comprised of a flexible blade for spreading the coating material uniformly on the web surface. The "Flexiblade" Coater made by The Black-Clawson Company is disclosed in Jacobs et al., U.S. Pat. No. 3,079,889 and in an article appearing in the Apr. 8, 1963 issue of Paper Trade Journal. It is also briefly described at p. 57 of the Apr. 29, 1963 issue of Pulp & Paper as well as other trade periodicals, both U.S. and foreign.
Other flexible blade coaters employing a closed or sealed, pressurized application chamber are described in U.S. Pat. No. 2,796,846 to Trist and U.S. Pat. No. 3,273,535 to Krikorian.
In another variant of the sealed chamber type of coater, coating liquid under pressure is extruded onto the web in the closed application chamber and an excess of coating is metered onto the traveling web by a metering bar at the rear or outgoing end of the chamber and the excess is then removed and the coating leveled to its final coat weight thickness by an inverted trailing blade engaging the web downstream from the motoring bar. Patents describing coaters of this type include Galer, U.S. Pat. No. 3,192,895;, Hunger U.S. Pat. No. 3,486,482 and Nagler U.S. Pat. No. 3,518,964. Of the three, the patent to Hunger U.S. Pat. No. 3,486,482, is the most representative.
The closed chamber type of coaters suffered the problem of excessive web breaks due to engagement of the traveling paper web with the mechanical sealing means required at the incoming, front or upstream end of the closed application chamber. Efforts to alleviate the problem, for example, by the use of flexible blade seals, such as those of Trist, or by spacing the Jacobs et al. seal member slightly from the web as suggested in the literature, failed to cure the problem. As a consequence, closed chamber coaters, including the Black-Clawson "Flexiblade" Coater, have been substantially if not entirely replaced by subsequent developments in paper coating technology. The above described variant thereof, as represented by the patent to Hunger, is not known to have been used commercially at all.
A fourth generation of blade coater, which was introduced by Black-Clawson as a replacement for the "Flexiblade" Coater, is characterized by an inverted trailing blade preceded by a fountain applicator which, like a dip roll, applies an excess of coating liquid to the web, which excess is subsequently removed and the coating leveled to its desired thickness by the trailing blade. Apparatus of this type, which are called Fountain Blade Coaters, are described in the Mar. 13, 1967 and May 13, 1968 issues of Paper Trade Journal (at pp. 52-53 and 64-67, respectively) and in a paper presented by Black-Clawson at a Tappi conference in 1978, and are disclosed in detail in the patents to Phelps et al. U.S. Pat. No. 3,418,970, Penkala et al. U. S. Pat. No. 3,453,137 and Coghill U.S. Pat. No. 3,521,602. A competitive apparatus, employing a jet applicator rather than a fountain applicator, is described in the German periodical Das Papier, No. 7, 1972, pp. 332-338, at page 334. Similar disclosures appear in an article by Ing. Josef Geistbeck, appearing in the German publication Walzen Und Glattschaberstreichanlagen, and in German Auslegeschrift No. 2359413.
With these prior art fountain and jet applicators, the amount of excess coating that is delivered to the trailing blade is purportedly metered onto the web by a metering or overflow strip which is located at the downstream edge of the applicator and adjustably spaced from the surface of the web to accomodate the escape of coating liquid between the web and the overflow strip. In use, these coaters encounter difficulties when running at high speed because the web catches on the metering bar and tears, thereby producing web breaks and causing machine down time and loss of production.
Some prior art coaters inherently employ a relatively long coating liquid dwell or soak time on the web, i.e., the time interval between the initial application and final blading of the coating. As a result, the water portion of the coating composition, as well as the water soluble or dispersable materials contained therein, migrate into the moving web at a more rapid rate than the pigment and eventually cause an undesirable imbalance in the coating constituents and their rheological properties. Long soak periods are also incompatible with the application of successive wet coats without intervening drying, i.e., wet on wet coatings, because the successive coat tends to migrate into and contaminate the previous coat.
In an effort to control soak time, Black-Clawson introduced a variation of its fountain blade coater wherein the fountain applicator and the doctor blade are separate assemblies and are relatively adjustable toward and away from one another in order to vary the dwell time of the coating on the web between application and doctoring. This coater, called the Vari-Dwell Coater, is described in the proceedings of the Tappi 1986 Blade Coating Conference, pages 109-113, and the Tappi1987 Coating Conference, pages 141-149.
The problems associated with long dwell times are discussed in U.S. Pat. No. 3,348,562 to Neubauer, who discloses a coater wherein a narrow stream of viscous coating is extruded onto an inverted trailing blade that defines a nip region with the roll supported web. Since the coating is bladed immediately after application, soak times are purportedly kept to a minimum. However, the coating application is such that the coating material is unpressurized after leaving the orifice and is supported on the blade or trailing side only, with the leading side of the stream being unsupported and exposed to the environs in the zone of application. Consequently, the coating material is not properly or uniformly applied to the web. Disclosures of a related nature are contained in U.S. Pat. No. 3,484,279 (FIG. 3) to Clark et al. and U.S. Pat. No. 3,070,066 to Faeber.
The fifth generation of blade coater comprises the short dwell time application coater or "SDTA" coater which is rapidly replacing the prior art blade coaters. In essence, the closed chamber, flexible blade, fountain blade and jet applicator coaters have been rendered obsolete, and the puddle and roll type coaters are being relegated to web precoating or prewetting functions in wet on wet coating systems. The short dwell time or "SDTA" coater is disclosed in detail in U.S. Pat. No. 4,250,211, and its advantages are discussed in the May 1984 issue of Pulp & Paper, pages 102-104.
The "SDTA" coater is characterized by a coating application chamber having a very small dimension in the direction of web travel, a doctor blade pressure loaded against the coated web at and defining the downstream or web outlet end of the chamber, a novel liquid seal formed within a fairly generous gap defined between the applicator and the web at the upstream or web inlet end of the chamber, and means for supplying coating liquid to the chamber under pressure and in such copiously excess quantities as to cause a continuous high volume flow of coating liquid through the gap out of the upstream or front end of the chamber in a direction opposite to the direction of web travel, thereby to form and maintain a liquid seal within the gap and to maintain the coating liquid under pressure in the chamber and as it is applied to and doctored off the web; the doctoring occuring immediately at the downstream end of the application zone while the coating liquid is maintained under pressure. The flow of excess coating liquid through the gap defined between the web and the front edge of the application zone, in the direction reverse to the direction of movement of the web, is such that the gap is continuously and completely filled with reversely flowing coating liquid in quantity sufficient to: (a) close and seal off the gap at the front edge of the zone to maintain the pressure application of the coating liquid to the web within the application zone; (b) strip air off the web as it approaches and enters the application zone, thereby to eliminate air induced skips and voids in the layer of coating applied to the web and insure uniform overall coating of the web; (c) prevent entrainment of air in the coating liquid in the application zone and in the coating liquid that is applied to the web, thereby to eliminate coating imperfections due to the presence of air bubbles in the coating on the web; (d) prevent entry of foreign matter through the gap into the application zone and the coating liquid therein; and (e) continuously clean and purge the application chamber and application zone to insure the integrity, homogeneity and uniform distribution of a continuously fresh supply of coating liquid within the application zone, and to ensure that no foreign matter or impurities, e.g., lumps or coagulated coating, reach the doctor blade where they could cause scratching of the coating or create other problems deleterious to the coating process, or result in web breaks.
Due to the facts that the moving web of paper is pressed firmly, continuously and tightly against the surface of the backing roll by the reversely flowing liquid seal at the front or web entry end of the application zone, by the pressure of the coating liquid within the application zone, and by the pressure loaded doctor blade at the rear or web exit end of the zone, the web cannot catch or snag on coater components and the web breaking and other disadvantages of prior art coaters are eliminated. Consequently, coating compositions can be applied to the web under pressure within a short dwell time, free of skips and voids even at very high web speeds. The SDTA coater has proven itself in use at speeds up to 4000 feet per minute ("fpm") and beyond to apply a more uniform layer of coating onto a web than any prior art coater.
Characteristics of the applied coating can be varied or enhanced by precoating the web, e.g., by a roll applicator as shown in U.S. Pat. No. 4,250,211 and improvement patent, U.S. Pat. No. 4,310,573, or by use of an internal leveling blade as disclosed in improvement patent, U.S. Pat. No. 4,369,731, or by use of a second, internal liquid seal as disclosed in improvement patent U.S. Pat. No. 4,452,833, or by use of other improvements of note such as disclosed in U.S. Pat. Nos. 4,396,648, 4,440,105, and 4,503,804.
A proposed variation on the SDTA coater, one version of which is disclosed in FIG. 3 of Wohlfeil patent, U.S. Pat. No. 4,706,603, involves essentially closing off the gap between the coater and the web at the upstream or web inlet end of the coating application chamber and draining excess coating from the chamber via drain holes in the upstream or front wall of the application chamber; the rate of drainage being such as to maintain the coating liquid in the chamber under pressure and to insure a sealed relationship between the web and the coater at the web inlet end of the application zone.
Another variant, a version of which is disclosed in U.S. Pat. No. 4,963,397 to Michael A. Mayer et al., involved utilization of a short dwell type of apparatus to rework a previously applied excess layer of coating liquid, e.g., a dip roll applied excess layer, to distribute over the web a more uniform layer of the coating; specifically, a layer of coating that is free of the film split pattern of dip rolls when operated at speeds above about 2,500 fpm; the blade of the short dwell coater being used to remove excess coating from the web and to smooth and level the coating to the desired wet film thickness and coat weight; the excess coating removed by the blade being drained away via the SDTA, e.g., in a manner such as disclosed in Wohlfeil. For another variant, see also U.S. Pat. No. 4,859,507 to Wayne A. Damrau.
While the SDTA, including the above-described variation and variants thereof, has significantly advanced the state of the art, it has not provided a final solution to all the expectations of the paper coating industry. As the industry presses forward to attain even greater capacity, efficiency and economy in the production of coated papers, even the SDTA coater has on occasion produced coated papers that would not satisfy the increased demand for high quality coatings at higher web speeds. In particular, when applying heavier weight coatings, for example, 51/2 and more pounds per side per 3,300 square foot ream, to the higher grades of paper webs, e.g., groundwood free merchant grades, at ever increasing production speeds, SDTA coatings, though of substantially uniform thickness and free of skips and voids, have exhibited decreased surface smoothness and streakiness in the direction of web travel through the coater, i.e., so-called machine direction or "MD" streakiness. Precoating or prewetting the web or reworking a previously applied excess coating on the web will not eliminate these problems. Dip roll applicators in particular encounter their own inherent limitations at web speeds in the order of 2,800 fpm due to splitting of the film of coating liquid being applied by the roll, resulting in a nonuniform coating having a longitudinally streaked or striped appearance, i.e., film split pattern.
While the SDTA coater and the above-described variants thereof can in most instances eliminate the film split pattern of the dip roll coating, MD streakiness and/or unacceptably diminished surface smoothness, i.e., surface roughness, may still result. Thus, whether used alone or in combination with a dip roll applicator, existing apparatus and methods, when operated at higher speeds to apply heavier weight coatings, may not in all cases produce a coated paper that will satisfy the exacting demands of the high quality printing, graphic arts and publishing trades.